I'm sure one of the scientist here can explain, but my limited knowledge of thermal cycling is you should avoid it (thermal cycling). As a manufacturer, we do such things to weed out problems with our product; either workmanship issues or defective parts. Everything sees some thermal cycling. Most handle things well as long as you stay away from extremes. Once you go from one extreme to another, you either need "beefy" things like cast iron or specialty products like Pyrex. Think of what happens with the items we use to cook with. One of the reasons boots take a beating is because while skiing, they'll go from our warm car, to the cold slope to the warm car, to the cold basement/garage etc. and that can be a temperature swing of 50 to 70 degrees. Most plastics don't like that and will be stressed. This stress will eventually cause cracks. Better plastics last longer. The activity level the plastics are seeing while thermal cycling will also affect how long they'll last. Someone riding the tails of their skis isn't stressing their boots near as much as a racer or hard carver would.
Simmer in AZ you're going through the same temperature swing but at a higher temp; 140 to probably 75-85 over night. What I don't know is what your gear can sustain for how long. Depending on the materials it could be no impact for several years to skis premature delamination over one summer.
The things in your favor are you aren't using them while they are thermal cycling. Way less stress on them. My non scientific mind thinks your biggest issue is going to be epoxies and glues weakening. I doubt any of the metals will care one way or another. I think the priority of concern would be (remember I'm guessing and have no data):
- Glues and epoxies
By structure I'm referring to things like camber and how the different parts continue to interact. The CTE for each material is going to be different so they will expand and contract slower and quicker. This is what can break them down and you'll get things like delamination and possibly even a softer ski. I'm really guessing at this point as I know these things occur,but I don't know at what temp for the materials you have or how long they can handle it.
I keep my gear in my basement and the coldest it ever gets is the low 40's and usually no hotter than 75. My issue is dealing with the humidity so I run a dehumidifier all summer.
I would also think that for your garage to get to 140F and needs to be hotter than that outside or you would need some other source of heat generation; paint it black, run equipment that generates heat etc. Put a couple cheap thermometers out there and check at the hottest part of the day. I'm sure it's hot, but I remember living in Barstow CA and the temp drops when I went into the garage. It was still hot, but not as hot as being in direct sunlight. I wouldn't want to go in the attic though!
Or I could be completely wrong,